The amount of flashpower needed depends on the nature of what you will be shooting. In evaluating this, the important considerations are:

the distance from the light(s) to the subject(s)

the size of the subject and shooting environment

the desired aperture range and the ISO speeds involved

For small studio portraiture, these parameters are fairly known. You should always shoot at the lowest ISO settings available from your camera (typically ISO 100, but some cameras limit you to a minimum ISO of 200). If you are limited to ISO 200, you will need half as much power as ISO 100 cameras.

In most small portrait studios, the photographer will want to be able to shoot from one to three or four people at apertures ranging from f4 to f16, using umbrellas and softboxes at distances of two feet to eight feet or so. Studio flash units rated from 150 Ws to 300 Ws each (or more) provide ample output for this purpose. It is extremely important that your flash units have a wide adjustability of power levels. A 6 f-stop power variability range (from Full Power to 1/32 power) is ideal. Of course, it is very important to have flash units with accurate modeling lamps so that you can compose your pictures and avoid excessive trial and error.

If you are limited to ISO 200, 300 Ws may be too much power because you may have difficulty in lowering the power enough to capture those f4 shots with close placement of the lights - particularly if you have a limited power adjustment range on your lights. If this is the case, you will be forced to move the lights further from the subject than you might prefer, or forced to use other methods such as neutral density filters to reduce the light levels.

Some users prefer to use lights with different power levels in such studios (for example a 320 Ws light for main and 160 Ws lights for fill, etc.), but many find this problematic and prefer to use three or four lights of the same power for easier interchangeability.

Our experience is that far more users end up wishing they had bought lower power lights (like the AlienBees™ B400) rather than higher power.

The situation changes dramatically when you take your lights on location to shoot larger subjects such as wedding groups, location fashion work, architecture, etc. Here, you may need considerably more power and may often be striving for higher aperture numbers. Most of these applications suggest flash units in the 600 Ws range. If your work regularly spans both small studio and location shots, the versatile 9 f-stop power variability of the Einstein™ would serve your needs well. The White Lightning™ X-Series X1600 units may also be a good option as these units are switchable from 660 Ws to 165 Ws and maintain a 6 f-stop power variability range at either power setting, thus affording an overall range of from Full Power to 1/128 power.

Beyond this, when even higher amounts of flashpower are needed, combined with very fast flash durations and recycle times, the Zeus™ series is indicated.